Focal isolation monochromator employing accentuation of longitudinal chromatic aberration



United States Patent 3,185,021 FOC..!. EOLATION ltiONOCHROMATOR EM- PLOYING ACCENTUAT ION 0F LONGITUDI- NAL CHROMATIC ABERRATION James S. Thompson, Los Angeles, Calif., assignor, by mesne assignments, to Thompson Raina Wooldridge Inc., Cle eland, t'llhio, a corporation of Ohio Filed Mar. 27, 1951, Ser. No. 98,640 2 Claims. (Cl. 88-14) This invention relates to a monochromator, and more particularly to a monochromator capable of greater spectral purity than previously capable by other such devices.

Monochromators generally have been constructed of a single positive lens and utilizing a point source of light to provide the desired longitudinal chromatic aberration for frequency selection through the use of a movable shield having a small pinhole. It can be seen, however, that with such systems the spectral purity is limited by the size of the frequency selecting pinhole and the speed of the lens. In addition, light from the point source passing along the axis of the lens contains all frequencies and thus passes through the frequency selecting pinhole.

It is therefore an object of this invention to provide a monochromator that is substantially single-frequency selective and also capable of utilizing a faster lens speed.

It is another object of this invention to provide a simple structure capable of substantially single-frequency selection.

It is another object of this invention to provide a monochromator with a lens system capable of decreasing the spherical aaerration and increasing the chromatic aberration, whiie at the same time providing substantially single-freq: :ncy selection.

It is still another object of this invention to provide a monochromator capable of eliminating the passage of light along the axis of the lens, thus eliminating the unwanted frequencies normally occurring along this axis.

Other otjects, purposes, and characteristic features will become obvious as the description of the invention progrosses.

In practicing this invention in one embodiment of a monochrcrna -)1 device, there is provided a doublet lens system, a point source of light, and a frequency selective diaphragm with a pinhole therein. The elements are ar-- ranged and positioned to prevent any passage of light frequencies through the doublet lens and to focal points along the lens axis without refraction, while at the same time accentuating the longitudinal chromatic aberration for easier f equency selection.

In the figures of the drawing:

FIGURE 1 is a view of a normal positive lens illustrating spherical aberration action;

FIG. 2 is a view of a positive lens illustrating the longitud'nal chromatic aberration normally found therein;

FIG. 3 is a view illustrating an embodiment of the pres ent imention showing light frequency selection that is substantialiy free of spherical aberration; and

FIG. 4 is a view illustrating another embodiment of the presc;! invention showing an external light source supply.

In each. of the several views similar parts bear like refzrence characters.

in the illustration of FIG. 1, there is provided a lens 1 receiving light from a source (not iilown) from a position to th Eeft of the .ens as viewed i the light rays 2 experiencing spherical to the cur. sture of the positive lens l' feing focused at c'ill'erent points along an axis 3. The spherical aberration focal point of the lens 1 is at different locations 3,185,021 Patented May 25, 1965 depending upon the wavelength or frequency of the light rays as separated by the spherical aberration For example, if the white light rays 2 striking the lens 1 are broken up into the different wavelengths, it will be found that the blue frequency light rays focus at a shorter focal length than the red light rays, as illustrated by the light rays 20 and the light rays 2b, respectively. The aberration causing the longitudinal focal separation of the different wavelengths in this case is caused by the the curvature of the lens 1 and is basically undersirable.

In FIG. 2, longitudinal chromatic aberration is illustrated, and in this figure the lens 1 is again receiving light from a source (not shown) in the form of the light rays 2, and, due to the refraction of the light rays passing through the lens 1, the white light of rays 2 is separated into the separate wavelengths from the blue to the red, for example, illustrated by the rays 24 and 2b, respectively, again falling on different focal points along the longitudinal axis 3 of the lens 1. Although the spherical and chromatic aberrations have been shown separately for simplification, it should be understood that with a positive lens such as the lens 1, both spherical and chromatic aberrations would be present, causing some dispersion in the focal point of each wavelength.

Looking now at FIG. 3, it can be seen that a structure is herein illustrated in which spherical aberration is reduced to a minimum, and chromatic aberration has been increased to give a wider spread betwxn the blue and red wavelengths in order that easier and more accurate wavelength selected can be accomplished. In this device, the positive lens 1 is now provided with a negative lens 4 and a reflective surface 5 positioned on opposite side thereof. In addition, the point light source 6 is positioned along the focal axis 3 of the lens 1 and is capable of emitting its light rays 2 over subs antially the entire reflective surface 5 of the lens 1. The positive lens 1 and negative lens 4 placed together now form a doublet lens, and, if .he positive lens 1 is made of high dispersion flint glass and the negative lens 4 is made of low dispension crown glass, an increase in spectral purity is obtained at the focal point of each of the wavelengths, since the chro matic aberration is emphasized, causing a greater separa tion between the focal points of the diierent wavelengths. In order to provide the desired frequency selection, a diaphragm 7 having a small opening or pinhole 8 therein is positioned with the pinhole 8 along the focal axis of the doublet lens formed by the positive lens 1 and negative lens 4. The diaphragm 7 is then positioned to receive the desired wavelength established by the refraction of the rays by the doublet lens. As shown in this illustration, the diaphragm 7 has been positioned at the focal point of the rays 20 illustrating the focal point of the frequency showing the visible color green. In illustration, white light is emitted from the source 6, is refracted through its passage through the negative lens 4 and positive lens 1, and is reflected by the reflective surface 5, and further refraction takes place resulting in the color separation illustrated by the rays 2a, 2b, and 2a: to give a wide or sensitive chromatic aberration along the focal axis 3 of the lens system. With the point source 6 placed on the focal axis 3 of the lens system, only reflected light striking the lens system at an angle capable of causing refraction is received at the diaphragm 7, and thus the light received is substantially pure, that is, only of the desired wavelength. Frequency selection is provided by moving the diaphragm 7 in the direction of the arrow 9 to provide for different wavelengths, as desired. Associated with the diaphragm 7 in position to receive the light wavelength passed by the opening 8 is a detector or receiver 10 which may take the form of a frequency sensitive film. It is to be understood that an enclosure 11 and a firm support must be provided for the elements, with a movable support for the diaphragm 7, with the exact support not being shown, since this forms no part of the present invention. This support for each element can be provided in the normal manner except for the fact that the light source 6 is repositioned on the opposite side of the doublet lens from that normally found in monochromators. It is to be further understood that the light source 6 may be placed on the side of the doublet lens formed by the positive and negative lenses 1 and 4, respcctively (in a manner normally expected), if the reflective coating is removed. However, some loss of effectiveness would occur due to the passage of light directly along the axis 3 from the source to the pinhole 8.

The embodiment of FIG. 4 is the same as that of FIG. 3 except for the fact that the light source 6 is replaced by a reflective member or mirror 12 for receiving light from an external source (not shown). The mirror 12 receives diverging light from a concave lens 13, used to cause the divergence of the received light, with the light incoming to the lens 13 being substantially collimatcd, such as would occur with light from a celestial body.

The light divergence established by the lens 13 is such that the reflection of the light by the mirror 12 substantially covers the area of the doublet lens formed by the members 1 and 4. In addition, the mirror 12 substantially reduces the impurities passed by the opening 8 by blockirg the axial portion of the light emitted by the reflective surface 5 of the doublet lens.

While there have been described what are at present considered preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed in the appended claims to cover "11 such changes and modifications as fall within the true spirit and scope of the invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a focal isolation monochromator having a pre determined optical axis:

a compound lens system for folusing light rays of different wavelengths to different longitudinally spaced focal points along said axis;

input means for introducing light rays to said lens system from a position substantially along said axis on one side of said lens system;

means including a reflecting surface, positioned adjacent the other side of said lens system, for redirectin g said light rays once more through said lens system and thereby to said ditierent focal points;

and diaphragm means having an aperture therein posi- .tioned along said axis at the one of said focal points which corresponds to a selected wavelength; with said compound less system including convergent 5 and divergent retracting elements formed respectively of a high dispersion material and a relatively low dispersion material for substantially accentuating the normally occurring longitudinal chromatic aberration while simultaneously reducing spheral aberration to an acceptable amount to thereby provide improved discrimination against wavelengths other than said selected wavelength.

2. In a focal isolation monochromator having a predetermined optical axis:

a compound lens system for focusing light rays of different wavelengths to ditlerent focal points longitudinally spaced along said axis;

input means for introducing light rays to said lens systern from a position substantially along said axis on one side of said lens system;

means including a reflecting surface positioned contiguous to the other side of said lens system for directing said light rays reversely through said lens system and thereby to said focal points;

and diaphragm means having an aperture therein, positioned along said axis at the one of said focal points which corresponds to a selected wavelength, for discriminatively transmitting said selected wavelength;

with said compound lens system including positive and negative lenses formed respectively or" flint glass and crown glass, whereby spherical aberration is substantiaiiy eliminated and chromatic aberration is appreciably enhanced to provide improved narrowband wavelength selectivity of the rays transmitted through said aperture.

4/47 France. 3/27 Germany. 3/58 Germany.

JEWEIL H. PEDERSEN, Primary Examiner.

WILLIAM MISIEK, Examiner. so 

1. IN A FOCAL ISOLATION MONOCHROMATOR HAVING A PREDETERMINED OPTICAL AXIS: A COMPOUND LENS SYSTEM FOR FOLUSING LIGHT RAYS OF DIFFERENT WAVELENGTHS TO DIFFERENT LONGITUDINALLY SPACED FOCAL POINTS ALONG SAID AXIS; INPUT MEANS FOR INTRODUCING LIGHT RAYS TO SAID LENS SYSTEM FROM A POSITION SUBSTANTIALLY ALONG SAID AXIS ON ONE SIDE OF SAID LENS SYSTEM; MEANS INCLUDING A REFLECTING SURFACE, POSITIONED ADJACENT THE OTHER SIDE OF SAID LENS SYSTEM, FOR REDIRECTING SAID LIGHT RAYS ONCE MORE THROUGH SAID LENS SYSTEM AND THEREBY TO SAID DIFFERENT FOCAL POINTS; 